1. Field of the Invention
This invention relates to polyamides and is more particularly concerned with rapidly crystallizable compositions comprising blends of aromatic-aliphatic polyamides and crystallization promoters.
2. Description of the Prior Art
Amorphous polyamides, particularly those high in aromatic backbone content (for example see U.S. Pat. No. 4,072,665), while being very useful for molding thermoplastics and fiber formation, do suffer from the drawback of remaining in the amorphous form after solidifying from the molten state.
Generally speaking, crystallinity in polyamides is desirable not only in order to speed up demold rates but also to maximize polymer properties such as heat deflection temperature, solvent resistance, dimensional stability, stiffness, and the like. In the case of fibers, crystallinity improves the breaking strength (tenacity). Unfortunately, the additional steps normally required to achieve crystallinity, such as prolonged demold times, annealing, etc., of parts and fibers, result in increases in manufacturing times and higher production costs.
The art of increasing the crystallization rate for certain polymers through the use of specific nucleating agents or crystallization promoters is known. However, this art of crystallization promotion is an empirical one and the findings with one polymer system cannot, as a rule, be applied to a different polymer.
For example, at page 466 of The Encyclopedia of Polymer Science and Technology, Vol. 10, 1969, John Wiley and Sons, New York, N.Y., it is noted that silicas are typical nucleating agents for the nylon polyamides. Contrastingly, U.S. Pat. No. 4,323,493 discloses that a particular class of amides (i.e. polyamide-imide) cannot be nucleated with silicas but can be nucleated with talc.
A variety of additives have been disclosed for accelerating the crystallization of the polyalkylene terephthalates. For example, alkali metal salts of higher fatty acids have been disclosed in U.S. Pat. No. 4,368,286. U.S. Pat. No. 4,368,288 states that finely divided inorganic nucleants such as talc are not very efficient, and an efficient nucleant such as sodium benzoate causes marked degradation of polyesters (column 1, lines 35 to 40) and therefore these two materials are unsuitable. This reference discloses the use of particular ionizable metal salts of organic compounds which actually react with the polyalkylene terephthalates in causing the enhancement of crystallization.
In contrast to U.S. Pat. No. 4,368,288 cited supra, Axelrod et al in U.S. Pat. No. 4,401,792 actually teach the use of alkali metal salts of benzoic acid as well as ionomers to promote the rate of crystallization of polyalkylene terephthalates.
U.S. Pat. No. 4,404,161 discloses an injection molding process for polyethylene terephthalate wherein about 2 to about 30 percent by weight of a multi-phase composite interpolymer is used to lower the molding temperature (conversely to enhance crystallization rate) of the polymer.
The series of polyamides consisting of poly(4,4'-methylenediphenylene azelamide), poly(4,4'-methylenediphenylene sebacamide), poly(4,4'-methylenediphenylene undecanediamine), and poly(4,4'-methylenediphenylene dodecanediamide) is well known in the art. All of these polymers solidify to the amorphous state and remain there unless annealed or heat treated. For references to such polymers see U.S. Pat. Nos. 2,669,556; 3,408,334; 3,640,970 and 3,651,022 and the J. Polymer Sci. 10, Part A-1, p 1547, 1972.
We have now discovered that compositions comprising the above amorphous polyamides prepared by a specific process and using certain types of crystallization promoters described below are rapidly crystallizable from the molten to solid state.
The combination of properties possessed by the molded products from these compositions, including high temperature resistance, excellent tensile properties and heat deflection temperature (HDT) values, reduced brittleness (improved impact strength), and, particularly, an extremely rapid rate of crystallization, are highly unexpected.